The surface figure shape and precision of optics directly impacts their performance. Synchrotrons, high-energy density laser systems, EUV lithography equipment, and astronomy instrumentation are all limited in some way by the quality of available optics. State of the art mirror figuring techniques include ion-beam figuring, elastic-emission machining, deterministic polishing, and magnetorheological finishing. All of these systems operate in an iterative fashion whereby mirrors are measured off-line in order to generate a correction map, and then the figures are corrected with the preferred technique. This process is repeated multiple times until the mirror shape is within the required error budget.
While the surface figuring technique can be fast, throughput is universally limited by the steps involved with transferring between the processing setup and the measurement setup. Furthermore, most industrial applications physically separate their “open-air” metrology stations from the surface figuring instruments.
For any of the figure correction techniques, and in particular for ion-beam figuring, significant dwell time is needed for the processed optic to adjust to new ambient conditions when switching between processing and metrology. Figuring performance can also be affected by switching between two separate processing and metrology stations because registration errors between the two steps can accumulate; this is even more prolific when fiducial markings cannot be used and the mirror edges are ambiguous.
Fizeau interferometers are one device used for such measurement applications. However, current interferometers suffer from several deficiencies. In a typical Fizeau interferometry application, a reference surface (transmission surface) serves as the reference flat. An optical wavefront passes through this transmission flat in order to reflect off of the surface under test (“SUT”). This reflected wavefront is collected by the interferometer and processed in order to produce a surface map that is used for figure correction. This is called a two-surface cavity measurement.
Insertion of extra optical surfaces into the measurement system (such as a vacuum window) will create aberrations to this wavefront. Added aberrations due to re-trace error can be induced if an off-axis interferometer design is used. Furthermore, the reference flat requires precision tip and tilt adjustment and must be rotated in order to perform the most rigorous reference flat error subtraction, called a “three-surface test”.
Another issue that has been found in certain cases to cause measurement instability is ambient humidity. The hygroscopic nature of antireflection coatings on transmission flats creates surface film stress that fluctuates with humidity, adding curvature to the flat.